This invention is directed to the marine industry and in particular to multi-blade propellers.
The propulsion system on a boat is one of the most important aspects of boat design, yet least understood. There are variety of items that make up the propulsion system and numerous items that affect how well the propulsion system works.
The propeller remains the most critical aspect of the propulsion system. Shaft angle, boat trim, stern gear, boat weight, engine horsepower and gear ratio are but a few items that affect propeller performance and behavior.
A major concern of propeller design is the amount of vibration that the propeller will produce while under way. As a general rule, in order to minimize vibration the number of blades on the propeller should be increased. There is no particular limit to the number of blades a propeller may have however, costs increase with the number of blades while the gain in reduction of vibration decreases with each additional blade. A negative consequence of increasing the number of blades on the propeller is the progressive reduction of efficiency of the propeller while operating in reverse to back down the boat.
Another major concern in propeller design is cavitation. One of the most unpredictable conditions that affects propeller operation is cavitation. Cavitation is a partial vacuum caused by excessive propeller speed or loading. The vacuum causes bubbles to form and implode irregularly causing uneven pressure on both sides of the blades resulting in vibration that feels like an unbalanced or unequally pitched blades. Further, the force of imploding bubbles can actually pull materials off the surface of the propeller leading to pitting, uneven wear, and resulting in bad balance and additional vibration.
On higher speed vessels, those operating over 40 knots, shaft rpms frequently force the propellers into a condition that some cavitation is difficult to avoid. For this reason super cavitating propellers have been developed that are capable of operating at high speeds without cavitation. These high speed propellers have blades shaped so that the low pressure side of the blade where cavitation forms, is vented to the atmosphere making cavitation almost impossible. The super cavitating propellers, commonly referred to as surface piercing propellers, were typically found only on high speed boats.
The surface piercing propellers are designed to work when partially submerged, e.g. about half in and half out of the water. Typically, such propellers are mounted aft of the transom except in cases like Small U.S. Pat. No. 4,689,026 where the propeller operates in a tunnel.
A disadvantage to these types of propellers is their inability to provide sufficient reversing thrust. The blade shape required for high efficiency at speed in a super cavitating design inhibits reversing properties that are normal to the typical propeller. This is caused by two factors. First, the blade has a progressive pitch which means that the pitch gets progressively higher as it approaches the trailing edge of the blade. When used in reverse, the trailing edge has too much pitch for efficient operation. Second, the trailing edge of a super cavitating propeller is sharp because, in forward, it is desired to have the flow of water separate from the blade efficiently. While required in forward, this sharp trailing edge becomes the leading edge in reverse and, as such, degrades reverse thrust by causing a ventilation bubble. If the blades are close together, the bubble from one blade can extend to the adjacent blade causing a total loss in reverse thrust. Yet a high number of blades is desired to minimize vibration so an inherent design conflict exists.
Thus what is lacking in the art is a multi-blade propeller having a shape that does not affect forward performance yet allows reversing properties similar to those of a conventional propeller.
The instant invention is directed toward a marine propeller with increased performance in reverse but without decreased performance in forward, having a hub and a multiplicity of blades extending radially outward from the hub. The separation of these blades about the hub lessens interference between the blades and increases the efficiency of the propeller. Interference between adjacent blades may be reduced by decreasing the number of blades or increasing the length of certain blades beyond the length of other blades or increasing the diameter of certain blades beyond the diameter of other blades.
Accordingly, it is an object of this invention to provide a multi-blade propeller with improved performance for backing down a boat.
It is a further object of this invention to decrease the interference of each propeller blade with the performance of the blades directly adjacent to it while operating in reverse.
Another object of this invention is to provide multi-blade propellers with a portion of the trailing edge of some of the blades further aft than the trailing edges of the other blades.
It is a further object of this invention to provide a propeller with blades having different widths.
It is a further object of this invention to provide a propeller with a modified trailing edge.
It is a further object of this invention to provide the trailing edge of the blades with a shallow concavity.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention. The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.